This PR introduces a new catalogues API for organising and accessing
related groups of sktime objects (datasets, estimators, metrics, etc.).
To understand the need and origin idea of catalogues, please check:
[https://hackmd.io/@jigyasu/rJkmyeGKgl](https://hackmd.io/@jigyasu/rJkmyeGKgl)
(Note that in the original design, we named this idea `collections` but
later on decided to go with `catalogues`)

### Changes:

#### New Base Class: `BaseCatalogue`

* `get()` method to retrieve items by category (dataset, forecaster,
classifier, metric) or all items
* `as_object` parameter in `get()` to return either names (strings) or
instantiated objects built from `registry.craft`
* caching for crafted objects
* `available_categories()` to discover collection structure
* Implements `__len__` and `__contains__` utlities
* Abstract `_get()` method for subclasses to define their items

#### Dummy Catalogues
* Dummy classification and forecasting catalogues